1. Field of the Invention
The present invention relates to a sheet feeding apparatus used with an image forming system such as a copying machine, printer, facsimile, word processor and the like, or other equipments utilizing sheets. More particularly, it relates to a sheet feeding apparatus for feeding a sheet (transfer sheet, photosensitive sheet, electrostatic recording sheet, print sheet, OHP sheet, envelope, post card, sheet original or the like) rested in a sheet containing portion such as a sheet supply cassette one by one to a sheet receiving portion such as an image forming station, exposure station, treating station or the like with a one-side reference.
2. Related Background Art
For convenience' sake, the related art will be described with reference to examples of laser beam printers shown in FIGS. 11A to 11C.
FIGS. 11A, 11B and 11C show laser beam printers having sheet paths, i.e., sheet feeding paths (extending from a sheet containing portion to a sheet ejecting portion) of straight type, U-shaped type and S-shaped type, respectively.
The printer having the sheet path of straight or linear type as shown in FIG. 11A has a sheet supply cassette inlet 51 formed in a side surface of a frame 50 of the printer, and a sheet ejection opening 62 formed in the other side surface of the frame. A sheet supply cassette (sheet containing portion) 52 is mounted within the inlet 51, and an ejection tray 63 is mounted within the ejection opening 62 and extends outwardly therefrom. When a sheet supply signal is emitted, a sheet supply roller 53 is rotated to afford a feeding force to an uppermost sheet (recording medium) P on a sheet stack rested in the sheet supply cassette 52, thereby separating the uppermost sheet from the other sheets with the aid of separating pawls 54 and feeding the sheet into the printer.
The fed sheet is conveyed between guide plates 55, 56 and through a path (sheet path) including a pair of register rollers 57, on image transferring portion 58, a conveyer belt device 59, an image fixing device 60, ejector rollers 61 and the ejection opening 62, whereby the sheet on which an image was formed (print) is ejected on the ejection tray 63.
An electrophotographic photosensitive member of drum type (referred to as "photosensitive drum" hereinafter) 64 is driven at a predetermined peripheral speed (process speed) around its own axis in a clockwise direction and is provided at its peripheral surface with a photosensitive body consisting of an organic or inorganic photoconductive layer. The reference numeral 65 denotes a charger for uniformly charging the peripheral surface of the photosensitive drum with the predetermined potential having a predetermined polarity; 66 denotes a beam scanner for scanning and exposing the charged surface of the photosensitive drum to write the aimed information thereon; 66a denotes a beam reflection mirror; 67 denotes a developing device for developing, with toner, an electrostatic latent image formed on the drum surface by the exposure; 68 denotes a transfer roller acting as a transfer means for transferring the toner image on the drum surface to the recording sheet P; and 69 denotes a cleaning device for cleaning the drum surface after the toner image is transferred to the recording sheet.
Since the principle and process for forming the image is well known, the explanation thereof will be omitted. Incidentally, in the illustrated printer, the photosensitive drum 64, charger 65, developing device 67 and cleaning device 69 are constituted as a single removable process cartridge 70.
In this printer, the sheet path extending from the sheet supply cassette 52 to the ejection tray (sheet ejecting portion) 63 is substantially straight, so that the reliability of the sheet feeding operation is increased. However, since the sheet supply cassette 52 and the sheet ejection tray 63 are protruded from both sides of the printer frame outwardly, the installation space for the printer will be greatly increased.
In order to reduce the substantial installation space of a printer, there has been proposed laser beam printers having sheet paths of U-shaped type or S-shaped type, as shown in FIG. 11B or FIG. 11C.
The printer having the sheet path of U-shaped type as shown in FIG. 11B has a sheet supply cassette inlet 51 and a sheet ejection opening 62 and is designed so that a sheet supply cassette 52 is wholly inserted into the printer from the inlet 51 and a recording sheet P supplied from the cassette by means of a sheet supply roller 53 is inverted by inversion guides 71a, 71b and convey rollers 72a, 72b to direct the sheet toward a direction opposite to a sheet feeding direction from the cassette 52 above the latter and is fed through a path including an image transferring portion 58, a conveyer belt device 59, an image fixing device 60, ejector rollers 61 and the ejection opening 62 and then is ejected onto an ejection tray 63. With this arrangement, since only the ejection tray 63 is protruded from the printer outwardly, the installation space for the printer is reduced in comparison with that for the printer of FIG. 11A.
On the other hand, the printer having the sheet path of S-shaped type as shown in FIG. 11C is designed similar to the printer of FIG. 11B, but the sheet ejected from the ejector rollers 61 is inverted again upwardly by an invertion guide 73 to be ejected onto an ejection tray 75 formed on a top plate of the printer by means of second ejector rollers 74. With this arrangement, since there is no member or element protruding from the printer outwardly, the installation space for the printer is further reduced in comparison with that for the printer of FIG. 11B.
Although the installation space for the printer can be reduced by forming the sheet path as the U-shaped configuration (FIG. 11B) or S-shaped configuration (FIG. 11C), the height of the printer will be increased in comparison with that of the printer having the straight sheet path as shown in FIG. 11A. To eliminate this drawback, there has been proposed to reduce the radii of the inversion guides as long as possible.
However, if the radii of the inversion guides are reduced, for example, in an inverting portion C in FIG. 11B, the sheet will be clogged or slacked between the inversion guides 71a, 71b due to the difference in speed between the convey rollers 72a, 72b, thus worsening the reliability of the feeding operation. Further, in the image forming system, when the recording sheet is not properly fed to the recording portion, the image formed on the recording sheet will be distorted.
Thus, there has been proposed that the sheet was fed by skew-feed rollers so that one lateral edge of the sheet was guided along a one-side feeding reference formed in an image forming system to prevent the skew-feed of the sheet. However, if the radii of the inversion guides for inverting the sheet are decreased, the skew-feed of the sheet will occur at the first skew-feed roller due to the feeding load in the inversion guide at a side opposite to the feeding reference side, thus damaging the edge of the sheet abutted against the one-side feeding reference.
Further, in view of the manufacturing technique, it is very difficult to completely coincide a position of a regulating member (formed on a sheet supply cassette) for regulating a lateral edge (at the reference side) of a recording sheet with a position of a feeding reference guide formed on an image forming system, with the result that the recording sheet will frequently be damaged. Explaining such inconvenience with reference to FIG. 12A, when the position of the regulating member 89 is deviated from the position of the feeding reference guide 90 by a distance .DELTA..sub.1, as the recording sheet 92 is fed while being biased toward a direction shown by the arrow by means of the skew-feed roller 91, the lateral edge of the sheet 92 will be bent or scratched at a position B in FIG. 12A.
On the other hand, as shown in FIG. 12B, when the feeding reference guide 90 is displaced inwardly from the position of the regulating member 89 by a distance .DELTA..sub.2, a corner of a leading end of the recording sheet 92 will be struck against the feeding reference guide 90 at a position C in FIG. 12C, thus scratching that corner.